The prior art discloses numerous paint delivery and application systems using pigging technology. Such systems may be used to apply seriatim paints of different colors, for example, to a plurality of substrates, such as vehicle bodies moving on a conveyor through a paint booth. As used herein, the term “paint,” includes solvent or water based paints used to paint or coat a substrate and generally any coating which may be applied to a substrate by an applicator including but not limited to a rotary spray atomizer. Depending on its function in the coating delivery system, pigs or pigging elements are variously referred to in the prior art as shuttles, separating elements, terminating pistons, plugs, etc. Generally, a pigging element is utilized in the prior art paint delivery systems to push paint from a source of paint, such as a color changer, through a delivery line to an applicator to apply paint to a substrate and the pigging element may also be utilized to return paint to the source of paint. As will be understood by those skilled in this art, a typical mass production application such as an automotive application, requires the application of different paints utilizing the same paint application system thus requiring purging of the delivery line and the applicator with solvent between applications. Typically, this requires a separate solvent delivery line to the applicator. The paint application system may include a first pig station adjacent the color changer and a second pig station adjacent the applicator. Paint is then delivered under pressure from the color changer to the delivery line and the pigging element is driven through the supply line generally by pneumatic pressure to push the paint in the supply line to the applicator and the pigging element is then received in the second pig station adjacent the applicator. The pigging element may also be utilized to return paint remaining in the supply line to the color changer by driving the pigging element from the second pigging station adjacent the applicator to the first pigging station adjacent the color changer generally under pneumatic pressure.
Finally, the prior art also includes the use of two or more pigging elements in the supply line which are utilized to separate a predetermined volume of paint or to provide an electrostatic barrier between volumes of paint which are being transmitted through the supply line particularly where the color change is at ground potential and the applicator is an electrostatic paint applicator at a greater electrical potential.
However, the prior art does not disclose a method or apparatus which eliminates the requirement for a separate solvent supply line to purge the applicator and the supply line and which may be utilized to return paint to the paint source.